The following publications are believed to represent the current state of the art:
U.S. Pat. Nos. 5,860,073; 5,909,568; 6,023,714; 6,157,935; 6,199,082; 7,047,033; 7,050,603 and 7,116,765;
U.S. Patent Application Publication Nos.: 2004/0093376 and 2005/0122997; and
Spyglass Prism, Concepts and Applications, Spyglass Inc., 1997, pp. 1-7.